Source drivers for driving a TFT (Thin Film Transistor)/LCD panel are of digital type and of analog type. According to the digital type, luminosity data corresponding to each pixel is delivered to a driver as a digital value and the driver latches the digital value and outputs the voltage corresponding to it. Methods for outputting the voltage are a switch scheme and a digital analog conversion (DAC) scheme. The switch scheme is performed by selecting and outputting one of a plurality of reference voltages. A driver using this scheme, can employ 4 bits (16 gradations) or 6 bits (64 gradations) requiring, but 16 or 64 switches for each output of the driver. It is impractical to implement gradations of more than 6 bits. The DAC scheme is performed by preparing a DAC for each output of the driver converting the received luminosity data into an analog value output. Disadvantages of this scheme are that it requires a large sized circuit and it is difficult to equate the performance of DAC provided for each output.
The output voltage of such a digital driver normally ranges from 0 V to 5 V. To cope with an LCD that is alternately driven between 0 V and 6 V and between 0 V and -6 V, a scheme of common inversion drive is adopted. This common inversion drive, changes the voltage of the common electrode of the LCD panel at a predetermined period (AC drive) as a result, the output of the driver appears to cover in the range of 1 V to 6 V and that of -1 V to -6 V (the range of 1 V to -1 V is a non-sensitive band). The period for which the voltage of this common electrode can be AC-driven is limited to the period of a horizontal sync. signal (H period). With this H inversion scheme, since a cross talk takes place in the horizontal direction of the screen, the deterioration of the screen presentation is inevitable.
With the analog type, conversion is not performed in each driver, luminosity data corresponding to each pixel is delivered to a driver in an analog value and the analog value is held in a sample hold circuit and outputted through a buffer amplifier. Since the analog type allows voltages of -6 V to 6 V can be outputted as a matter of course, it is unnecessary to perform the common inversion drive. While use of a high withstand voltage element leads to an increase in the size of each element, with proper circuit design, there is a good possibility that the total size can be made smaller than that of the digital driver. Furthermore, because of being able to cope with an infinite levels of gradations using the same circuit independently of the number of gradations, the analog driver is fit to implement more 256 gradations (full color). In addition, since it is not required to perform a common inversion drive, the HV inversion drive scheme which performs opposite-polarity write to neighboring pixels is implementable. Furthermore since no cross talk takes place, a high-quality image can be displayed.
While this, the analog type provides a high quality image, however significant design effort is required to suppress the variation of outputs and the occurrence of errors to sufficiently low limits. Furthermore, an output amplifier with a large power consumption is needed. As mentioned above, this is because outputs of from -6 V to 6 V are needed.
Accordingly, analog drivers have thus far not been utilized often for purposes having stringent requirements for power consumption, such as the display of a notebook PC, where used most of them have been large-sized drivers for high resolution displays, such as XGA or SVGA in a way to drive the source line of the LCD panel from the top and the bottom of the panel. In Published Unexamined Patent Application No. 6-295162, a scheme of driving by the two-side drive method is described.